The present invention relates to a cable system, and more particularly, to a system and method for feeding a compound to the entire cable system to enhance its performance.
Underground solid dielectric electrical cable technology was developed and implemented because of its aesthetic advantages, immunity from weather-induced failure, and its relative cost effectiveness compared to earlier generations of underground cable that used a solid-liquid dielectric, namely, paper and oil. Currently, underground solid dielectric electrical cables generally include a number of copper or aluminum strands surrounded by a semiconducting or insulating strand shield, a layer of solid dielectric insulation, and an insulation shield.
Underground solid dielectric electrical cables were initially touted as having a useful life of 25-40 years. However, the useful life of such cables installed before 1985 has rarely exceeded 20 years, and has occasionally been as short as 10-12 years. In particular, the solid insulation tends to degrade over time because water enters the cable and forms water trees. Water trees are formed in the insulation when medium- to high-voltage alternating current is applied to a polymeric dielectric (insulator) in the presence of water and ions. As water trees grow, they compromise the dielectric properties of the polymer until the insulation fails.
To address the water tree issue, the conventional wisdom has been to overdesign a brand-new cable. For example, a brand-new power cable is typically designed to have an AC breakdown strength of 800 to 1000 volts/mil, though with common insulation thickness only 400 volts/mil is actually required for a cable to reliably operate. This 2 to 2.5 times overdesign is required because the AC breakdown performance of a new cable begins to degrade as soon as the cable is installed and put in service. The cable industry has spent the last twenty years improving the materials and manufacturing techniques used in forming cables, in particular, the cable insulation and strand shield. This approach, however, increases the costs of manufacturing cables because it often requires expensive insulation materials.
Water tree growth can be eliminated or retarded by removing or minimizing the water or ions. One approach for accomplishing this in old cables is to continuously inject a desiccant fluid into the interstices between the strands of electrical cables. For example, U.S. Pat. No. 4,545,133 to Fryszczyn et al. describes a method of continuously injecting desiccant fluid, typically a dry gas, into one end of a cable and continuously flushing the water-rich fluid out of the other end of the cable. The fluid travels generally axially along the interstices of the cable. U.S. Pat. No. 4,372,988 to Bahder, U.S. Pat. No. 4,766,011 to Vincent, and U.S. Pat. Nos. 5,372,840 and 5,372,841 to Kleyer have taught methods to introduce liquid materials that diffuse radially from the strands of installed power cables to treat existing water trees. Bahder taught the use of non-water-reactive materials, while Vincent and Kleyer introduced an improved concept, which incorporated water-reactive functionality to dry the cable in a single step. The present treatment approaches, however, suffer from several disadvantages.
First, continuous axial flushing of desiccant requires that a substantial amount of desiccant be continuously fed to a cable. Equipment for continuous feeding must be frequently refilled or maintained, making the maintenance cost of such a system quite high.
Second, axial desiccation establishes a concentration gradient of water in the cable insulation with a very low value, near zero, adjacent the cable strands, but an increasing value at a point radially removed from the cable center. Axial desiccation, at best, can reduce the rate of water tree growth, but it cannot entirely eliminate it.
Third, continuous feeding of non-oligomerizing material will result in permeation or exudation of treatment material from the cable into the environment. Ensuing environmental damages and loss of treatment material are some of the undesirable results of such permeation or exudation.
Fourth, continuous, unrestrained feeding of non-water-reactive or water-reactive treatment materials may cause a condition of xe2x80x9csupersaturationxe2x80x9d, wherein the amount of treatment fluid that is delivered to the cable exceeds the amount of fluid required to optimally treat the cable. Supersaturation eventually swells the polymer material forming the cable to such an extent that the mechanical strain bursts the cable and it fails catastrophically.
Fifth, because a separate feeding system is required to feed each cable or small group of cables linked in series, a large number of such systems must be installed and maintained at substantial cost in order to treat the multitude of cables in a typical distribution cable circuit. Systems, which link a small number of cables in series, suffer the disadvantage that the effectiveness of the axial desiccation is compromised as moisture from the first cable is carried into subsequent segments.
Sixth, when a cable termination is enclosed in an insulating housing, or xe2x80x9cdead-frontxe2x80x9d device, feeding a desiccant fluid to such a dead-front termination compromises its safety characteristics.
Seventh, the prior art addresses the rejuvenation of previously installed cables and cannot be applied to brand-new cables for the purpose of altering their fundamental design to eliminate the need for overdesign.
The present invention provides a system and a method for overcoming all of these disadvantages.
The present invention provides a distributed feed system for use in a cable system, where the cable system includes at least two cable subsystems or segments. The distributed feed system is adapted to feed a performance-enhancing compound into each of the cable subsystems. The distributed feed system includes a central feed station having a tank for holding the performance-enhancing compound. The distributed feed system further includes an impermeable or low-permeable distribution conduit that connects the central feed station to each of the cable subsystems to permit distribution of the performance-enhancing compound therethrough to each of the cable subsystems.
In accordance with one aspect of the present invention, the central feed station further includes a flow control system coupled to the tank, which is adapted to controllably release the performance-enhancing compound from the tank. The flow control system may be an osmotic flow control system using a permeable membrane. The area of the membrane available for the compound permeation may be varied so as to control the compound permeation through the osmotic flow control system.
In accordance with another aspect of the present invention, one or more osmotic flow control devices may be placed along one or more cables forming the cable system to control diffusion of the performance-enhancing fluid through each cable and, hence, through the entire cable system.
In accordance with yet another aspect of the present invention, the distributed feed system further includes a communication network including a central database. The central feed station includes a data communication device for transmitting data, relating to the central feed station, to the central database via the network.
In accordance with still another aspect of the present invention, the distribution conduit for transporting the performance-enhancing compound is integrally formed with a cable. For example, one or more neutral wires within a cable may be replaced with one or more tubes to serve as distribution conduit(s).
In accordance with a further aspect of the present invention, the cable may include a permeable conduit axially extending therein. The permeable conduit is provided for purposefully carrying the performance-enhancing compound through the cable, and for permitting the compound to migrate generally radially through the permeable conduit into the cable. The distribution conduits of the distributed feed system may be advantageously coupled to these permeable conduits to efficiently transport the performance-enhancing compound to each of the cable subsystems.
In accordance with a still further aspect of the present invention, the material used to form the cable, including its permeable conduit, may be selectively chosen so as to control diffusion of the compound through the cable.
In accordance with yet a further aspect of the present invention, a shielded dielectric tube including a dielectric inner tube and a semiconducting or conducting outer tube surrounding the inner tube are provided. The shielded dielectric tube may be used to provide for a complete dead-front termination while safely feeding a performance-enhancing fluid into the termination.
In accordance with an even further aspect of the present invention, the performance-enhancing compound comprises a silane, which can be readily altered to control the permeation rate and the extent of oligomerization of the compound. By controlling the permeation rate and the extent of oligomerization of the compound, one may control the amount of compound delivered to the cable and the maximum extent of oligomerization of the fully hydrolyzed compound so as to mitigate the problem of supersaturation.
In accordance with yet a further aspect of the present invention, a cable may be designed without the overdesign universal to solid dielectric cables manufactured and installed today. Specifically, the present invention allows the dielectric performance of the cable to remain at or very near 1000 volts/mil for an indefinite period of time, thereby reducing the required insulation thickness by up to 60%. Further, the materials used for the solid dielectric and the shields in the cable system can be made of less expensive materials as the cable will remain totally dry over the lifetime of the cable. The present invention provides various advantages.
First, since multiple-cable subsystems, or cables, are fed with a performance-enhancing compound using a central feed station, the costs of maintaining the single central feed station are significantly lower than the costs of maintaining multiple feeding equipment as required in the past.
Second, because diffusion of a performance-enhancing compound through each cable and throughout the entire cable system is controlled by various means, continuous axial flushing of treatment fluid is not required. Axial delivery along the length of the cable is controlled by means such as osmotic flow control devices, while radial diffusion may be controlled by carefully selecting the materials forming the cable, such as the material used to form the permeable conduit provided within the cable. Radial diffusion may further be controlled by manipulating the chemical structure or the degree of oligomerization of the performance-enhancing compound. Controlled diffusion allows for only an optimal amount of performance-enhancing compound to be fed into the cable system, reducing accidental spill and waste. Further, controlled diffusion prevents a xe2x80x9csupersaturationxe2x80x9d condition.
Third, continuous feeding and controlled diffusion of the performance-enhancing compound, when applied in a new cable, allow for the insulation of the cable to maintain its initial dielectric properties substantially intact. This obviates the need to overdesign a new cable, and allows for the use of less expensive materials and particularly thinner insulation in constructing a new cable. Fourth, the shielded dielectric tube of the present invention allows for formation of a complete xe2x80x9cdead-frontxe2x80x9d termination while safely feeding a performance-enhancing fluid into the termination.